The above-mentioned communication system is used in a situation where it is necessary to keep transmission subject data transmitted between two communication apparatuses confidential to a third party. Although various encryption techniques are proposed and used in order to keep the transmission subject data confidential, it is difficult to completely prevent decryption of a code.
In general, communication performed by encrypting the transmission subject data is performed by using a procedure of cutting the transmission subject data by a predetermined number of bits on a transmitting and receiving apparatus on a transmitter side, encrypting each piece of the cut data and transmitting it to the communication apparatus at the other end and decrypting received data on the transmitting and receiving apparatus on a receiver side.
In the case of performing such encryption, an algorithm and a key which are predetermined are generally used. This algorithm is rendered very complicated to prevent the decryption of the code, and the key is also changed in predetermined timing in many cases. Once the algorithm and key are known, however, it is relatively easy to break the encrypted data by using the algorithm and key even if the algorithm is rendered complicated or the key is changed.
The inventors hereof studied encryption technology for many years and previously invented a technique for performing encrypted communication wherein each of the communication apparatuses on the transmitting side and the receiving side is provided with common means for successively generating at least one of the algorithm and key for performing the encryption and decryption so as to perform the encrypted communication while successively changing at least one of the algorithm and key used for the encryption and decryption.
This technique successively generates at least one of the algorithm and key for performing the encryption and decryption. Even in the case where the algorithm or the key is known once, the algorithm or the key or both of them change thereafter. Therefore, strength thereof is much higher than conventional encryption technology.
As for this technique, however, there are the cases where, if some of the past algorithms or keys are known, it is predictable as to how the algorithm or the key or both of them change from now on. Thus, a possibility of being broken by a third party is not absolutely zero.
An object of the present invention is to improve the communication system including two communication apparatuses capable of encrypting transmission subject data in plain text to render it as encrypted data and then transmitting it to the communication apparatus at the other end and also decrypting received encrypted data and rendering it as the transmission subject data so as to reduce the possibility of being broken by a third party.